\documentclass[12pt]{article} 
 \textheight6cm \textwidth11cm \parindent0mm \parskip 1ex plus2ex minus0.8ex
\begin{document}
\marginparsep 1ex  \marginparwidth 3ex

You may always have marginal notes appear within text.
\marginpar{\rule{1mm}{5mm}}

Usually marginal notes will be put on the right side, yet there are exceptions
(two-sided or double-column; in these cases \verb+\marginpar+ will be
positioned `outside', and this may be the left margin).
If you cannot predict where the marginal note will be positioned,
and if the form of the marginal note depends on its position (as it is e.g.
with these arrows)\marginpar[$\rightarrow$]{$\leftarrow$},
you should use the 2 argument form of the \verb+\marginpar+ command.

\reversemarginpar
This is also helpful in connection with \verb|\reversemarginpar|, 
as you may see here.
\marginpar[$\rightarrow$]{$\leftarrow$}
\end{document}
